In general, a wheel type working vehicle representative examples of which are wheel loaders, wheel type excavators and the like rotates wheels to travel on general roads toward a working site. In this case, there are wheel type working vehicles that drive a hydraulic motor by an engine (prime mover) and transmit the rotation of the hydraulic motor to wheels or are equipped with a power transmission device for vehicles that transmits the rotation of an engine through a torque converter and a transmission to wheels.
Here, the aforementioned power transmission device for vehicles includes a casing that is mounted on a vehicle, a transmission that is provided in the casing to change rotational speeds of the engine, a torque converter that is mounted in the casing to transmit the rotation of the engine through fluid to the transmission, and a lockup device that is mounted in the torque converter to transmit the rotation of the engine directly to the transmission.
The torque converter is configured of a rotating body that is rotatably supported in the casing and is rotated by the engine, and a hollow cylindrical transmission shaft that is rotatably supported in the casing to transmit the rotation of the rotating body through fluid or the lockup device to the transmission.
On the other hand, the lockup device is configured of a lockup clutch that establishes connection between the rotating body and the transmission shaft by pressurized oil delivered from a hydraulic pump, a lockup control valve that controls supply/discharge of the pressurized oil to/from the lockup clutch, and a lockup oil passage that introduces the pressurized oil from the lockup control valve to the lockup clutch (Patent Document 1).